If a bag yields 0.012 m³ of mixed concrete, 0.5 m³ with a 10% allowance requires ceiling(0.5 × 1.10 / 0.012), or 46 bags. The yield must come from the product specification.
Why this works
Measure length, width, and thickness in the same unit. Multiply them for volume, convert to the unit used by the bag yield, apply your chosen allowance, then round the number of bags upward. A 4 m × 2 m × 0.10 m slab has a geometric volume of 0.8 m³.
When this answer changes
Bag mass measures the dry product; mixed yield measures the resulting volume. Different mixes can have different yields even at the same bag mass. This quantity estimate does not specify reinforcement, strength, foundation design, or structural suitability.
Try your own values
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